Different mutations have different target genders which they tend to push the player towards. In most cases, this is pushing the player towards their own gender, though not in all cases. This does result in a trend though where female creatures tend to make you female and males tend to make you male. And while there are plenty of males that aim to make the player female, there are fewer females striving to make the player male. For reference, checking the Wiki will show you the target gender of any given creature.

Now, with all that said, there is a simple means for you to maintain control of your gender if you wish to keep it fixed throughout the game and that is by fixing it during start-up using the 'Gender Lock' option. It is setting 7 during character creation. This will allow you to dictate what gender your character will maintain during the game... mostly. A few situations that occur will supersede any selection made here, but those are rare.

If you're already in a game and do not wish to restart, you can also find the 'Vet Supplies' event at the zoo. This event allows you to set yourself to add/remove the 'Male Preferred', 'Female Preferred' or 'Herm Preferred' feats. Using this method will eliminate the 'Single Sexed', 'Always Cocky' and 'Always a Pussy' feats, should you have them. Eventually this event will be upgraded to have flexibility closer to the start-up menu's options, but that's a lower priority update and likely won't occur for a while.

There is also a third option, should you dare to take it, and that is by consorting with dark forces in the form of the Hellhound. I don't really want to spoil what happens, but suffice it to say that it's a one-way trip and that you should take this option only if you're okay with starting a fresh game later. That said, this path does have a lot of hidden Easter Egg endings when coupled with many of the automatic game over scenarios out there.